Chapter 14: I/O Systems
I/O Hardware
Common concepts ：
- Port
- Bus (daisy chain or shared direct access)
- Controller (host adapter) 操作端口和总线的设备
Devices have addresses, used by ：
- Direct I/O instructions
- Memory-mapped I/O
Polling：
Determines state of device：
- command-ready
- busy
- Error
Busy-wait cycle to wait for I/O from device（忙等待）
Direct Memory Access
- Used to avoid programmed I/O for large data movement
- Requires DMA controller
- Bypasses CPU to transfer data directly between I/O device and memory
Application I/O Interface
Kernel I/O Subsystem
Scheduling ：
- Some I/O request ordering via per-device queue
- Some OSs try fairness
Buffering ：
store data in memory while transferring between devices
- To cope with device speed mismatch
- To cope with device transfer size mismatch
- To maintain “copy semantics”
Caching -：
fast memory holding copy of data
- Always just a copy
- Key to performance
Spooling（假脱机）
hold output for a device
- If device can serve only one request at a time
- i.e., Printing
Device reservation -provides exclusive access to a device
- System calls for allocation and deallocation
- Watch out for deadlock
I/O Protection
User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions.
- All I/O instructions defined to be privileged
- I/O must be performed via system calls
Performance
I/O a major factor in system performance:
- Demands CPU to execute device driver, kernel I/O code
- Context switches due to interrupts
- Data copying
- Network traffic especially stressful
Improving Performance:
- Reduce number of context switches
- Reduce data copying
- Reduce interrupts by using large transfers, smart controllers, polling
- Use DMA
- Balance CPU, memory, bus, and I/O performance for highest throughput
DMA原理
DMA原理：DMA 是所有现代电脑的重要特色，他允许不同速度的硬件装置来沟通，而不需要依于 CPU 的大量 中断 负载。否则，CPU 需要从来源把每一片段的资料复制到暂存器，然后把他们再次写回到新的地方。在这个时间中，CPU 对于其他的工作来说就无法使用。 DMA 传输重要地将一个内存区从一个装置复制到另外一个。当 CPU 初始化这个传输动作，传输动作本身是由 DMA 控制器 来实行和完成。典型的例子就是移动一个外部内存的区块到芯片内部更快的内存去。像是这样的操作并没有让处理器工作拖延，反而可以被重新排程去处理其他的工作。DMA 传输对于高效能 嵌入式系统 算法和网络是很重要的。
在实现DMA传输时，是由DMA控制器直接掌管总线，因此，存在着一个总线控制权转移问题。即DMA传输前，CPU要把总线控制权交给DMA控制器，而在结束DMA传输后，DMA控制器应立即把总线控制权再交回给CPU。
后记：至此，OS的主要课程就结束啦，感谢zwshen老师井井有条的线上教学，以及学长和同学们的帮助。很欢迎大家和我讨论关于OS的问题呀！联系方式 [email protected]